Neuropathic pain can be caused by various underlying diseases, such as viral infections and diabetes. For example, post herpetic neuralgia is caused by herpes viral infection and typically causes moderate to severe pain in the infected skin area to the subject. Topical products, such as creams or patches containing appropriate drugs, may be used to control neuropathic pain; however, patches and traditional semisolid formulations such as creams and ointments both have significant shortcomings. Semisolid formulations usually contain solvent(s), such as water and ethanol, which are volatile and thus evaporate shortly after application. The evaporation of such solvents can cause significant decrease or even termination of dermal drug delivery, which can be undesirable in many cases. Additionally, semisolid formulations are often “rubbed into” the skin, which does not necessarily mean the drug formulation is actually delivered into the skin. Instead, this phrase often means that a very thin layer of the drug formulation is applied onto the surface of the skin. Such thin layers of traditional semisolid formulations applied to the skin may not contain sufficient quantity of the active drug to achieve sustained delivery over long periods of time, which can be desirable in treating neuropathic pain. Additionally, traditional semisolid formulations are often subject to unintentional removal due to contact with objects such as clothing, which may compromise the sustained delivery and/or undesirably soil clothing.
A patch containing an appropriate drug can be used to treat neuropathic pain. However, subjects often have to cut the patch to fit the shape and size of the skin area to be treated, which is inconvenient. Another shortcoming of patches is that they are usually neither sufficiently stretchable nor flexible for every application location because the backing film (in matrix patches) and the thin fluid bag (in reservoir patches) are typically made of polyethylene or polyester, both of which are relatively non-stretchable materials. If the patch is applied on a skin area that is significantly stretched during body movements, such as joints and muscles, separation between the patch and skin may occur, thereby compromising the delivery of the drug. In addition, a patch on a skin surface may hinder the expansion of the skin during body movements and cause discomfort and/or aggravate pain. For these additional reasons, patches are not ideal dosage forms for skin areas subject to expansion and stretching during body movements.
In view of these and other shortcomings, it would be desirable to provide systems, formulations, and/or methods for treating neuropathic pain that i) can provide sustained drug delivery over long periods of time; ii) are not vulnerable to unintentional removal by contact with either clothing, other objects, or with other people for the duration of the application time; iii) can be applied to a skin area subject to stretching and expansion without causing discomfort or poor contact to skin; and/or iv) can be easily removed after application and use.